FIG. 4 is a cross section depicting a structure of a conventional cooling device employing a motor having a dynamic-pressure-fluid-bearing. FIG. 5 is a cross section of a motor-bearing employed in the cooling device of FIG. 4.
A structure of the prior art is described hereinafter with reference to FIG. 4 and FIG. 5.
Housing 141 having one open side and being cup shaped is protrusively formed on a recess of frame 101. Housing 141 secures stator 103 on its outer wall, and stator 103 is wound with coil 102. Driving circuit substrate 104 is disposed around housing 141. Substrate 104 holds stator 103 and connects electrically a terminal of coil 102 to a wiring formed on substrate 104 by soldering. Substrate 104 is equipped with electronic components constituting the driving circuit and Hall elements. Insulating sheet 120 is disposed between substrate 104 and frame 101.
Frame 101 is surrounded by a side wall and has an upward opening. Bell-mouth 119 is disposed around the opening to promote airflow. Thrust plate 107 made of resin is disposed on a bottom face of housing 141. Sleeve 105 is fit into housing 141. Stator unit 115 comprises these elements discussed above, i.e. frame 101, housing 141, sleeve 105, coil 102 and stator 103.
Rotary shaft 109 extends through sleeve 105 and is axially underpinned by thrust plate 107 as well being rotatably supported by sleeve 105. Fan 108 is mounted to shaft 109. Magnet 111 is bonded to fan 108 via magnet yoke 112 so that magnet 111 faces stator 103. Rotor 116 comprises the elements discussed above, i.e. magnet 111, yoke 112 and fan 108.
The bearing of the motor is detailed hereinafter with reference to FIG. 5.
In FIG. 5, sleeve 105 is equipped with oil reservoir 147 near the center of its inner wall. Oil reservoir 147 has a greater inner diameter than other parts of the inner wall of sleeve 105. Sleeve 105 has dynamic-pressure-generating grooves 113 on both sides of oil reservoir 147. Grooves 113 are formed by a ball-rolling-process. Oil 114 is provided to grooves 113 for sleeve 105 and shaft 109. Radial bearing 117 is thus formed as discussed above.
The tip of shaft 109 facing thrust plate 107 is processed into a spherical face that contacts thrust plate 107 so that thrust plate 107 supports shaft 109 axially. A thrust support 106 independent of frame 101 supports the thrust plate 107. Thrust bearing 118 is thus structured as discussed above.
Reference number 300 represent a heating load on the cooling device positioned on a face of the cooling device opposite to the face of the frame forming the recess.
The conventional motor employing this dynamic-pressure-fluid-bearing, however, has the following problems.
Electronic apparatuses including personal computers and electric home appliances have been downsized in recent years, which entails requiring cooling-fan-motors, one of the components of the apparatuses and appliances, to be smaller and slimmer. In order to meet this requirement, a bearing space is narrowed, which forces the outer diameter of rotary shaft 109 and inner diameter of sleeve 105 to be narrowed.
In the prior art, since a bite shank having ca. 2 mm diameter has been used in processing the bearing, oil reservoir 147 can be processed with regard to sleeve 105 having over 3 mm inner diameter. However, when sleeve 105 is downsized to have not more than 2 mm inner diameter, the bite shank must have a diameter not more than 1 mm in order to form the oil reservoir. The shank having a diameter not more than 1 mm encounters abnormal vibrations due to the narrowed body when the sleeve is processed, and is broken frequently. If the process speed is slowed down to avoid this breakage, process time increases, which boosts the manufacturing cost. Narrowing of the inner diameter of the sleeve has thus been at a standstill from the standpoint of processing.
Apparatuses and appliances which are equipped with more functions and have undergone the downsizing process are obliged to liberate a greater heating value. The cooling-fan-motor mounted in these apparatuses and appliances experiences significant temperature changes, and is forced to drive at a high rotational speed in order to promote cooling efficiency.
Oil 114 provided between shaft 109 and sleeve 105 produces surface tension, and oil 114 retained in dynamic-pressure-grooves is moved along the grooves to the centers thereof by the spin of shaft 109, thereby producing a pumping force.
The high rotational speed increases the centrifugal force and the temperature changes widely, which increases expansion and contraction of oil 114 per se, and air entrapped in the oil 114. Due to these expansions and contractions, oil 114 overflows the bearing and travels along shaft 109 and rotor 116 to the outside of the motor. Due to this oil-spill, abnormalities are found such as an oil shortage at the bearing, a lower number of rotations, increased electric current, abnormal sounds, and further, a locked rotor.